Presently available data processing systems which are often referred to as belonging to the "mini-computer" class normally handle logical addresses and data words which are 16 bits in length. As used herein, the term "logical" address, sometimes referred to by those in the art as a "virtual" address, is used to denote an address that is programmer visible, an address which the programmer can manipulate. In contrast, a "physical" address is the address of a datum location in the main memory of a data processing system. Operating data processing systems utilize appropriate translation tables for converting logical addresses to physical addresses.
Such systems require appropriate handling of accesses to the main memory and require access both by internal system components and external (I/O) components.
Accordingly, it is desired that such data processing systems have means for controlling the access and for providing the most effective and efficient techniques for accessing not only specific data words but also blocks of data words.